Hypoxia and Medicine - Why do jet air crafts fly at such high altitudes?

Why do jet air crafts fly at such high altitudes?

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It's more fuel efficient to fly at the higher altitude (in the jet stream) because it's like paddling down stream in a river, less effort.
well the higher you are the easier it is to go faster than the speed of sound so im guessing the same concepts apply to why they fly so high, maybe :D
That's because it gives the best fuel efficiency (min fuel required for a distance traveled) and jet aircraft cruise at Mach No. It's a comparison of the aircraft speed to the speed of sound. The higher it flies, the outside air temperature reduces thus the local speed of sound reduces, so the aircraft won't have to struggle that much to achieve the optimum cruising speed. And thus less struggle means less fuel burned. The third things is, for aviation height and speed means better chance of survival. If there's an engine failure or any emergency, the higher we are, the more chances for us to identify and regain control of the aircraft. If there's an engine failure or total engine failure, the higher we are, the more choices and will be able to glide to further suitable fields or airport for force landing provided the best glide speed for the aircraft had been maintained.
The higher you go the smoother the flight , you are above the turbulence & you use less fuel. Also you don't get passengers parachuting out & complaining about the airlines' food.
Primarily because jet engines are much more fuel efficient at higher altitudes. The jet routes start at 24,000 feet. Sometimes jet pilots will complain if they are held at lower altitudes to long by air traffic controllers as it uses up much more fuel. Other benefits are you are generally well above the clouds and the air is smoother although there can be air turbulence, which is sometimes severe.
The simplest way to understand it, IMO, is like this: Imagine two jets. One is at 5,000 feet and the other is at 35,000 feet. Assume both jets are experiencing the same aerodynamic conditions. Because the air is much thinner at 35,000 feet, the jet at 35,000 feet would have to be going much faster to have the same aerodynamic conditions as the jet at 5,000 feet. You can think of the air at 5,000 feet as a bit like water compared to the air at 35,000 feet. It's much thicker, and therefore lower speeds will produce higher forces. The main downside for commercial jets is that at higher altitudes, the speed of sound is lower. Unless you're talking about the Concorde, you can't exceed the speed of sound. In fact, as you get higher, the minimum safe speed (have to go fast enough to produce lift) and the maximum safe speed (can't exceed the speed of sound) start to get closer and closer together, creating the "coffin corner". Pilots that get too high may be unable to tell if they're going too fast or too slow -- get it wrong, and your correction will kill you.
As you climb, the air becomes thinner. The air near the surface is pressed by the weight of all of the air above it, making it denser. Thinner air offers less resistance to the plane. Have you ever tried to run in a swimming pool? It takes more work than running out of the water, because water is denser than air. The same principle applies - it takes less work to move though thinner air. Pilots fly the plane in reference to the indicated airspeed, which is a measurement of the pressure against the front of the plane caused by it's movement. Pilots flight plan and navigate using the true airspeed - how fast the plane is actually flying though the air. The indicated airspeed may be the same at 5000 ft as at 35000 ft, but at 35000 ft the plane is traveling much faster though the thinner air to create that same amount of pressure against the airspeed sensor. In both cases the airspeed indicator may read 250 kt, but at 35,000 ft the TRUE airspeed would be hundreds of kts faster. A couple of points on the other replies: Yes, it is possible to get a huge tailwind flying with the jetstream, but that only works in one direction. An airplane may see increasing headwinds as it climbs, but the effects of the headwinds rarely offset efficiency gained by climbing. Yes, the speed of sound decrease, but that is a limiting factor. Planes do not become more efficient because they are flying at a higher mach number - just the opposite. As you approach mach1,parts of the local airflow around the plane will exceed Mach 1. This transonic range produces extra drag. The mach number is a limiting factor, at high altitude a plane will hit it's limiting mach number below it's maximum indicated airspeed. For a U2 at 80,000 ft the critical mach number is only 10 kts above the planes stall speed. It's flying at a high mach number, but flying just about as slow as it can.
Faster, more fuel efficient and you can avoid most of the weather.
The higher an airplane flies, the less fuel it burns. It requires more power to fly high, but the air is thinner, so you can go a lot faster, and overall less fuel is burned at high altitudes than at low altitudes. That's the main reason for flying high. Another secondary reason is that flying high puts an airplane above most weather and turbulence, making for a smoother ride that is less likely to require course changes to avoid bad weather.
People have worked out a certain altitude where flying is most efficient in terms of fuel, passenger comfort and overall cost of maintenance, through looking at weather cells/patterns, jet stream location and laws of physics which keep the aircraft in the air. This is called cruising altitude, normally 35,000 feet for jet airliners.
Air traffic controller is the one who is giving assign altitudes to every departing aircraft. it depends also to the type of aircraft…Every aircraft that is flying have different altitudes to avoid collision. Pilots could not decide by their own altitudes but they can request …higher altitudes is less density, pressure temperature, and air is thinner due to low density.
1) Because they can! 2) Better fuel efficiency because there's less drag on the airframe. 3) Less birds & other traffic. 4) Smoother air. 5) Can take advantage of jetstreams. 6) Altitude is your friend. It's safer in case something goes wrong. 7) The view is better.
to avoid storms, save fuel, and avoid clouds for a turbulent flight
short and simple version......saves fuel, less turbulence, and helps there to be less conjusted airspace at lower altitudes.